Small aerial side lobes in height and laterally are desired in radar systems for several reasons, e.g. to minimize the action of active jamming transmitters or strong ground echoes (clutter). For a conventional aerial, e.g. a reflector aerial, the side lobe level is dimensioned from the appearance of the illumination function. For an electrically controlled aerial (ESA) with a plurality of active transmitter-receiver modules the phase and amplitude similarity between the different modules will be dimensioning for the side lobe level.
In the literature the mean side lobe level is generally given according to EQU SL.sub.M =SL.sub.O +(.sigma..sup.2 /.eta..sub.N)
where
SL.sub.O =the side lobe level due to the illumination function PA1 .eta.=aerial efficiency PA1 N=number of modules PA1 .sigma..sup.2 =amplitude-phase error between transmitter-receiver modules.
If it is desired to realize, for example, 40 dB side lobes with say 200 modules, this means a similarity requirement of &lt;0.5 dB and some degrees between the modules, which is very difficult to achieve with today's technique, particularly if performance shall be maintained over a large temperature range. Each module must consequently be calibrated in a simple manner during operation, i.e. a calibration signal with known phase and amplitude must be fed to each of the aerial elements, and the same signal must be measured on the output of the respective module. The difficulty here is in being able to feed a calibration signal of sufficient accuracy to a sufficiently large number of modules.
It is already known to connect an extra calibration aerial into the remote or immediate field of the existing radar aerial, so that the signals of the extra aerial are received/transmitted through the radar system. It is also known to feed a test signal behind the aerial via wave conductors, coaxial, stripline or microstrip transmission and to evaluate the signals from the different elements, with the object of getting the amplitude and phase of the different signals as uniform as possible.